Liquid-in-liquid printing of 3D and mechanically tunable conductive hydrogels

Conductive hydrogels require tunable mechanical properties, high conductivity and complicated 3D structures for advanced functionality in (bio)applications. Here, we report a straightforward strategy to construct 3D conductive hydrogels by programable printing of aqueous inks rich in poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) inside of oil. In this liquid-in-liquid printing method, assemblies of PEDOT:PSS colloidal particles originating from the aqueous phase and polydimethylsiloxane surfactants from the other form an elastic film at the liquid-liquid interface, allowing trapping of the hydrogel precursor inks in the designed 3D nonequilibrium shapes for subsequent gelation and/or chemical cross-linking. Conductivities up to 301 S m−1 are achieved for a low PEDOT:PSS content of 9 mg mL−1 in two interpenetrating hydrogel networks. The effortless printability enables us to tune the hydrogels’ components and mechanical properties, thus facilitating the use of these conductive hydrogels as electromicrofluidic devices and to customize near-field communication (NFC) implantable biochips in the future.

The presence of PEDOT:PSS in the aqueous phase alone does not exhibit strong interfacial activity, while the amine end-capped silicone oil (PDMS-NH2) in the oil phase possesses a low degree of interfacial activity (as indicated by its IFT value at the steady state). When both components are present in each phase simultaneously, the interfacial tension decreases dramatically and reaches a low value. It is reasonable to conclude that the amine-capped silicone oil initially assembles at the interface in the form of PDMS-NH3 + , and the negatively charged PEDOT:PSS particles then quickly diffuse to the interface and electrostatically interact with the amine endgroups on the silicone oil to form the PPSs interfacial assembly. c The droplet morphology and its buckling behavior when the PEDOT:PSS dispersion is withdrawn. Well-defined wrinkles are only observed at the interfacial area with PPSs assembly.
Supplementary Fig. 2 | pH-dependent PEDOT:PSS-PDMS surfactant interfacial assembly (PPSs). a Temporal evolution of the interfacial tension (IFT) of aqueous PEDOT:PSS dispersions (0.5 mg mL -1 , pH range of 3.01-11.82) introduced into solutions of PDMS-NH2 in toluene (10 vol%). b. Droplet morphologies of PEDOT:PSS dispersion (0.5 mg mL -1 , pH 1.21-2.55) introduced into solutions of PDMS-NH2 in toluene (10 vol%). At pH lower than 2.55, the initial adsorption of PEDOT:PSS particles to the interface is too rapid to form regular droplet shapes for IFT measurements. Adjusting the pH further to 1.21, the PEDOT:PSS droplet became opaque and parts of the assembly containing PEDOT:PSS were removing from the aqueous phase into oil. This may arise from the formation of micelles (water in oil (W/O)) at the liquid phase, due to the very low interfacial tension and osmotic pressure at low pH. 1 Supplementary Fig. 3 | Variation of zeta-potential of PEDOT:PSS aqueous solution with pH. Supplementary Fig. 4 | A continuous jet without breakup when introducing a PEDOT:PSS dispersion (0.5 mg mL -1 , pH 2.07) into a solution of PDMS-NH2 in toluene at a flow rate of 0.07 mL min -1 .
Supplementary Fig. 5 | The ability of instantaneous interfacial PPSs to maintain the original shape of droplets or liquid threads. a) A pendent PEDOT:PSS droplet (1 mg mL -1 aqueous dispersion) in toluene solution of PDMS-NH2 surfactants (10 vol%) reaching its steady state within milliseconds and maintaining this state for at least 12 hours. b) A printed PEDOT:PSS thread (0.5 mg mL -1 aqueous dispersion) in oil phase (10 vol% PDMS-NH2 in silicone oil, ηe ~ 30000 mPa·s) maintaining its shape for 12 hours.
Supplementary Fig. 6 | Visualizing the PPSs interfacial assembly. a Formation of a planar layer of PPSs at the liquid-liquid interface in a mold. When the aqueous and oil phases are brought into contact, interfacial assembly occurs, resulting in the formation of a flat and uniform PPSs film at the interface. The assembled layer remains stable in water due to the cross-linked structure, allowing it to be easily transferred to a glass substrate. b PPSs assembly layer collected on glass substrate. c SEM images showing the densely packed PEDOT:PSS nanoparticles at the assembly layer. Similar results were observed in two independent samples. d Size distribution of PEDOT:PSS particles by intensity, measured at ~pH 3.5 using dynamic light scattering (DLS).
Supplementary Fig. 7 | Diameter control of printing pure PEDOT:PSS threads in oil phase with various viscosities. a Optical images of the PEDOT:PSS aqueous dispersion (10 mg mL -1 ) injected in various silicone oil containing 10 vol% PDMS-NH2 surfactants. b Diameter distribution of the printed PEDOT:PSS aqueous dispersion (10 mg mL -1 ) in different silicone oil with various viscosity. The diameter of the printed PEDOT:PSS threads in cylindrical shape is only related to print head speeds (ν) and ink volumetric flow rate (Q) but not to the oil viscosity. The threads thickness can also be calculated by the following formula: Supplementary Fig. 13 | Comparison of different drying treatments on printed hydrogels (ink 5): lyophilization, lyophilization followed by annealing, and direct dry annealing. a Polymer morphology changes of the printed hydrogels (ink 5) after drying treatments. To eliminate other influencing factors, the printed hydrogel samples were directly dried without removing the uncured monomers. While some degree of pore collapse is observed for all three drying treatments, the directly dry-annealed samples show a higher degree of network stacking and aggregation, which is also illustrated in the diameter variation of the samples. The similar results were observed across two independent samples within each group in a. b Changes in mass, diameter, and electrical conductivity of the printed hydrogels during different drying treatments. While the amount of water loss is almost the same for all treatments, it's reasonable to conclude that the water-loss-induced collapse of the conducting PEDOT@PEG network into aggregates in the dry annealing process is the main contributor to the enhanced conductivity of the printed gels. Values in b, c, and d represent the mean, and the error bars represent the SD of the measured values (n = 3).
Supplementary Fig. 14 | Conformational changes of the PEDOT@PEG gels after cyclic stretching. a Schematic illustration of molecule orientations in edge-on and face-on configurations, with respect to the direction of polymer gel stretching. b 2D GIWAXS measurements of printed PEDOT@PEG gels (ink 5*) before and after reversible stretching of the gel between 0 and 50% strain. The printed PEDOT@PEG gels exhibit the similar characteristic peak to the pristine PEDOT:PSS material. c Ascription of face-on and edge-on regions by the χ angle: edge-on region (0° < χ < 30°), face-on region (60° < χ < 90°). d Intensity ratio change of face-on to edge-on orientations. To investigate the molecular orientation, sector integrals from horizontal (0°-30°) to vertical (60°-90°) direction were calculated for the printed PEDOT@PEG gel before and after cyclic stretching to 50% strain. As scattering from the π-π stacking of the PEDOT:PSS molecules in horizontal and vertical directions characterizes the edge-on and face-on orientations of the molecules relative to the stretching direction, respectively, the PEDOT:PSS crystallites changed from a disordered arrangement to a preferred face-on molecular orientation during stretching, as evidenced by the noticeable increment of the face-on to edge-on integration ratio. e Raman spectra illustrating the Cα=Cβ vibration peak shift for the PEDOT@PEG gels (ink 5*) after reversible stretching treatment. This shift indicates a higher proportion of the benzoid moieties in PEDOT have been converted to the quinoid structure via oxidative charge transfer doping, and such conversion leads to a more planar backbone, contributing to more efficient charge delocalization and higher packing order. 3,4 Supplementary Fig. 15 | Electrical stability of the printed gels in liquids. a Resistance change of the gels (ink 5, ink 5*, ink 6, ink 6*) when soaked in DI water for certain days. b Gel ink 6* showing good electrical stability over 7 days. c The longterm electrical stability of gel ink 5* and ink 6*. Both post-treatment (comparation between ink 5 and ink 5*, or between ink 6 and ink 6*, shown in a) and ionic liquidinduced gelation (comparation between ink 5* and ink 6*, shown in c) are capable of improving the electrical conductivity and stability of the resultant gels considerably. 1cm-long cylindrical samples were used for measuring resistance under various conditions. Values in b represent the mean, and the error bars represent the SD of the measured values (n = 3).
Supplementary Fig. 16 | Gelation of liquid PEDOT ink 5 when mixed with varying amounts of ionic liquid (IL) HOOCMIMNTF2. The ink 5 containing 2 wt% IL undergoes gelation at a moderate rate, whereas the one consisting of 5 wt% IL has an irreversible phase separation after 10 min. The similar results were observed across two independent samples within each group.
Supplementary Fig. 17 | Flowability of the PEDOT ink when mixed with 2 wt% ionic liquid HOOCMIMNTF2 (ink 6). 200 µL fresh ink 6 was injected into a glass tube and left to sit for a specified time period. The tube was then tilted at a 45-degree angle to observe the ink's flow behavior. The similar results were observed across two independent samples within each group. Given that the inside diameter of the tube is 1.5 mm, the capillary force is negligible compared to the gravity of the injected ink liquid. When the physical gelation time is less than 10 minutes, the ink in the tube remains in a liquid state and flows under the action of gravity, with some liquid remaining on the tube wall. However, when the gelation time is prolonged to 15 minutes, the ink appears as a bulk and loses its fluidity in the glass tube. Based on this observation, we conclude that the ink mixture can maintain its flowability for at least the first 8 minutes after adding the ionic liquid. Supplementary Fig. 23 | Evaluation of the anti-adhesive effect of PEDOT@PEG hydrogels (ink 5, ink 5*, ink 6 and ink 6* formulas) with and without PPSs interfacial assembly on HEK 293T cells. a Schematic showing the fabrication of PEDOT@PEG hydrogels without PPSs interfacial assembly in a mold and subsequent cell seeding on hydrogel surfaces. b Morphology of HEK 293T cells in direct contact with PEDOT@PEG hydrogels without PPSs interfacial assembly after 72 hours of culture time. c Schematic showing the fabrication of PEDOT@PEG hydrogels with PPSs interfacial assembly in a mold and subsequent cell seeding on hydrogel surfaces. d Morphology of HEK 293T cells in direct contact with PEDOT@PEG hydrogels with PPSs interfacial assembly after 72 hours of culture time. At the end of the 72-hours culture, the difference in cell spreading between the hydrogel and the surrounding petri dish demonstrates that both PEDOT@PEG hydrogels with and without PPSs interfacial assembly possess anti-adhesive effects on cells. The similar results were observed across two independent samples within each group in b and d.
Supplementary Fig. 24 | In vivo assessment of hydrogel biocompatibility in immunocompetent mice. a,b Schematic diagram and photograph depicting the subcutaneous implantation of printed hydrogels (ink 6*), which is the formula that contains the most components. c Pathological study through H&E staining on the implantation site 7 days after surgery. d-f Representative immunohistochemistry (IHC) staining of markers for neutrophils (LY6G), pan-macrophages (F4/80), and M2 macrophages (CD206), respectively. The blue line indicates the hydrogel-tissue interface. The ratio of M2 macrophages to pan-macrophages (> 66%) around the hybrid PEDOT@PEG hydrogel suggests that the macrophages mainly display alternative activation and, therefore, exhibit an anti-inflammatory response, supporting tissue repair. The printed PEDOT@PEG hydrogels exhibit high biocompatibility in immunocompetent mice at this time point, as the mice remained alive and healthy without any abnormalities for two months after subcutaneous implantation. Longerterm monitoring of the foreign body reaction in the mice is still ongoing. The similar results were observed across four independent samples within each group in c, d, e, and f.